Metal fatigue is the damage or failure of a metal structure evidenced in the form of cracks. It primarily caused by cyclic tensile loads acting on the structure. Fatigue cracks generally start at the location of highest stress within the structure. Typically, this occurs at a change in section of the structure. A typical change in section would be a fillet radius, bend, hole, notch or other cutout. The change in section causes the otherwise uniform tensile stresses to be concentrated at that location, thus providing a potential weak spot in the structure. Geometric features that increase the local stress in this manner are called stress risers or stress concentrators. By far the most common stress concentrator is a fastener hole. This is easy to understand, since fastener holes concentrate the uniform stress by a factor of three or more at their periphery.
One of the most successful methods for improving the fatigue life of holes in metal structures is a process called cold working. The term cold working covers a number of processes that impart beneficial residual compressive stresses around the hole. In this disclosure, the term cold working will be used broadly to describe three cold-working methods each used for treating holes in structures, namely split sleeve cold working, split mandrel cold working, and StressWave™ cold working. In each of the aforementioned cold-working methods, the improvement in fatigue life is produced by surrounding the hole with a large zone of residual compressive stress in the remaining material structure. Such residual compressive stress reduces the magnitude of the tensile stress at the hole, thereby increasing the fatigue life of the structure.
In the case of both the split sleeve and split mandrel cold working methods, the compressive stresses are imparted into the material bounding the hole by pulling a tapered mandrel through the hole. Basically, the action of pulling the mandrel through the hole plastically expands the wall of the hole and the surrounding bounding material. Material further not immediately at the hole edge wall is also deformed, but to a much lesser extent. The elastically expanded material attempts to return to its original position, as it existed prior to the cold working, but such movement is resisted due to the permanently expanded hole edge wall. As a result, the spring back of the elastic material induces a compressive residual stress around the hole wall.
A forerunner of hole cold working was the mandrel only process. That process required access to both sides of the structure to be worked, since the mandrel diameter was larger than the starting hole diameter. Prior to mandrel insertion a lubricant was manually applied to both the hole and the mandrel to reduce pull force and galling. The attaching end of the mandrel, sticking through the hole, was attached to a pulling device. The pulling device, typically a hydraulic tool, pulled the mandrel through the hole. The treated hole was then drilled to the desired final diameter.
The split sleeve process was developed to improve the efficiency of hole cold working by making it a one-sided process. Also, it allowed an increase in applied expansion by using solid film lubricant coated split sleeve thereby increasing fatigue life; see U.S. Pat. No. 3,892,121. The use of a dry-film lubricant applied only to the mandrel contacting side of the sleeve eliminated the requirement for applying lubricants manually. The one-sided split sleeve process reduced cold working labor hours significantly. Additionally, the sleeve protected the hole against scoring and galling caused by the sliding contact of the mandrel. Undesirably, though, an axial ‘ridge’ is left in the hole as a result of the split in the sleeve, and that ridge needs to be machined out of the hole in most cases.
The split mandrel process, also a one-sided hole cold-working method, was developed to eliminate the disposable split sleeve; see U.S. Pat. No. 4,164,807. The system features a collapsible hollow mandrel and a high film-strength liquid lubricant, typically cetyl alcohol. A solid pin in the center of the collapsible mandrel is used to provide support for the mandrel during expansion. Before mandrel insertion into the hole the center pin is retracted and the mandrel collapsed. After the mandrel is inserted into the hole, the center pin is then pushed into the mandrel to make it “solid”. A spray mist of liquid lubricant is applied automatically to the hole just before the mandrel is pulled through.
The split sleeve and split mandrel hole cold working methods are used routinely in assembly of new and the rework of existing structures. For production and assembly of new aircraft structures with cold worked holes, the currently utilized method is to pre-assemble the various components such as a wing skin and a stringer using tack fasteners, drill the required undersized starting hole through each of the layers in the assembly, then cold work the hole. After cold working, the hold is then drilled to the desired final diameter. Since the starting hole passes through all the layers of an assembly, the cold working tooling can treat all layers, for a single hole, in the same operation.
Rework of an existing structure the process is very nearly the same as just described for new holes in aircraft structures. In a rework operation the existing hole is reamed up to a proper starting hole diameter for cold working. The starting holes in each layer of the assembly are cold worked in the same cold-working operation. After cold working, the hole is reamed up to its desired final diameter. The diameter of the as cold worked hole is typically about 0.015 inches less than the desired final diameter. Because of the tooling and manpower involved in the setup, initial drilling, cold-working, and final reaming steps, the cost of cold working at the assembly level is rather high. As a result, it would be desirable to eliminating cold working during the final assembly process.
The foregoing figures, being merely exemplary, contain various elements that may be present or omitted from actual implementations of the method(s) disclosed, depending upon the circumstances. An attempt has been made to draw the figures in a way that illustrates at least those elements that are significant for an understanding of the various embodiments and aspects of the invention. However, various other elements of the exemplary methods provided, especially as applied for different applications and variations of the fatigue life enhancement processes described, may be utilized in order to provide an advantageous manufacturing method for providing fatigue life enhanced products such as aerostructures.